1. Field of the Invention
This invention relates to sintered silicon nitride bodies useful as materials for parts required to have strength, especially excellent impact strength for items such as automobile parts and machine parts.
2. Description of the Related Art
There has been rapid progress in recent years in developments toward improved fuel consumption in the automobile-related field to cope with the global environmental problems of CO.sub.2, SO.sub.x and NO.sub.x and the like. With a view toward enhancing efficiency by reducing weight, friction loss or the like, there has been interest in using a ceramic material as a valve system material. Silicon nitride materials in particular are most promising candidates for their light weight, high strength, high toughness and high Young's modulus. For actual use, these silicon nitride materials are, however, still not fully satisfactory in mechanical properties such as strength and toughness. Examples of the applications of ceramic materials to such automobile parts include an impeller for a turbocharger [Shibata, Hattori and Kawamura, "Niyu Seramikusu (New Ceramics)" 1, p 91 (1988)] and a tappet shim for a diesel engine [Hara, Kobayashi, Matsui and Akabane, "Jidosha Gijutsu (Automobile Technology)", 45(4), p. 33 (1991)]. These examples both make use of the characteristic light weight and high abrasion resistance of silicon nitride ceramics, but their strength is as low as 100 kg/mm.sup.2 in terms of 3-point bending strength as measured in accordance with JIS R1601. These materials are not considered to have sufficient strength to be usable for valve systems or mechanical parts, especially in the area of impact strength where they have a potential problem in reliability. Especially, parts such as tappet shims referred to above, when employed in a conventional reciprocating engine, must have high impact resistance as they are subjected to very high impact stress in a non-steady operation state (for example, during surging), to say nothing of abrasion resistance. For the use in such parts, there has been a strong demand for of a high strength silicon nitride material having a superior impact resistance.